Activation of platelets and the resulting platelet aggregation and secretion of factors by the platelets has been associated with different pathophysiological conditions including cardiovascular and cerebrovascular thromboembolic disorders, for example, the thromboembolic disorders associated with unstable angina, myocardial infarction, transient ischemic attack, stroke, atherosclerosis and diabetes. The contribution of platelets to these disease processes stems from their ability to form aggregates, or platelet thrombi, especially in the arterial wall following injury or plaque rupture.
Platelets are known to play an essential role in the maintenance of hemostasis and in the pathogenesis of arterial thrombosis. Platelet activation has been shown to be enhanced during coronary thrombolysis which can lead to delayed reperfusion and reocclusion. Clinical studies with aspirin, ticlopidine and a monoclonal antibody for platelet glycoprotein IIb/IIIa provide biochemical evidence for platelet involvement in unstable angina, early stage of acute myocardial infarction, transient ischemic attack, cerebral ischemia, and stroke.
Platelets are activated by a wide variety of agonists resulting in platelet shape change, secretion of granular contents and aggregation. Aggregation of platelets serves to further focus clot formation by concentrating activated clotting factors in one site. Several endogenous agonists including adenosine diphosphate (ADP), serotonin, arachidonic acid, thrombin, and collagen, have been identified. Because of the involvement of several endogenous agonists in activating platelet function and aggregation, an inhibitor which acts against all agonists would represent a more efficacious antiplatelet agent than currently available antiplatelet drugs, which are agonist-specific.
Current antiplatelet drugs are effective against only one type of agonist; these include aspirin, which acts against arachidonic acid; ticlopidine, which acts against ADP; thromboxane A.sub.2 synthetase inhibitors or receptor antagonists, which act against thromboxane A.sub.2 ; and hirudin, which acts against thrombin.
Recently, a common pathway for all known agonists has been identified, namely platelet glycoprotein IIb/IIIa complex (GPIIb/IIIa), which is the membrane protein mediating platelet aggregation. A recent review of GPIIb/IIIa is provided by Phillips et al. (1991) Cell 65: 359-362. The development of a GPIIb/IIIa antagonist represents a promising new approach for antiplatelet therapy. Recent studies in man with a monoclonal antibody for GPIIb/IIIa indicate the antithrombotic benefit of a GPIIb/IIIa antagonist.
There is presently a need for a GPIIb/IIIa-specific antiplatelet agent which inhibits the activation and aggregation of platelets in response to any agonist. Such an agent should represent a more efficacious antiplatelet therapy than the currently available agonist-specific platelet inhibitors.
GPIIb/IIIa does not bind soluble proteins on unstimulated platelets, but GPIIb/IIIa in activated platelets is known to bind four soluble adhesive proteins, namely fibrinogen, von Willebrand factor, fibronectin, and vitronectin. The binding of fibrinogen and von Willebrand factor to GPIIb/IIIa causes platelets to aggregate. The binding of fibrinogen is mediated in part by the Arg-Gly-Asp (RGD) recognition sequence which is common to the adhesive proteins that bind GPIIb/IIIa.
Several RGD-containing peptides and related compounds have been reported which block fibrinogen binding and prevent the formation of platelet thrombi. For example, see Cadroy et al. (1989) J. Clin. Invest. 84: 939-944; Klein et al. U.S. Pat. No. 4,952,562, issued Aug. 28, 1990; European Patent Application EP 0319506 A; European Patent Application EP 0422938 A1; European Patent Application EP 0422937 A1; European Patent Application EP 0341915 A2; PCT Patent Application WO 89/07609; PCT Patent Application WO 90/02751; PCT Patent Application WO 91/04247; and European Patent Application EP 0343085 A1.
In the present invention we use conformationally-constraining carbocyclic ring systems as templates for cyclizing peptides such that they have high affinity and selectivity for GPIIb/IIIa.